The present invention relates to cylindrical superconducting magnets, and in particular to arrangements for locating such magnets within a housing. Many superconducting magnets are housed within a cryogen vessel, and are cooled by partially filling the cryogen vessel with a liquid cryogen, such as liquid helium, which boils and holds the magnet at the boiling point of the cryogen. The magnet must be firmly attached to the cryogen vessel. Other arrangements are known, in which no cryogen vessel is provided. In such arrangements, the magnet is housed within an outer vacuum container (OVC). The present invention is principally directed to arrangements for attaching a cylindrical magnet structure to a cryogen vessel.
FIGS. 1A-1B illustrate cross-sectional and axial sectional views, respectively, of a conventional cylindrical magnet arrangement for a nuclear magnetic resonance (NMR) or magnetic resonance imaging (MRI) system. A number of coils 34 of superconducting wire are wound onto a former 1 to form a cylindrical magnet structure. The resulting assembly is housed inside a cryogen vessel 2 which is at least partly filled with a liquid cryogen 2a at its boiling point. The coils 34 are thereby held at a temperature below the critical temperature at which they become superconductive. Commonly, the liquid cryogen 2a is helium, and this holds the coils 34 at a temperature of about 4K.
The former 1 is typically constructed of aluminium, which is machined to ensure accurate dimensions of the former, in turn ensuring accurate size and position of the coils on the former. Such accuracy is essential in ensuring the homogeneity and reliability of the resultant magnetic field. The formers must therefore be very rigid and firmly retained in position, relative to the bore tube 8 or cryogen vessel 2, in order to accurately locate the homogeneous imaging volume. Support protrusions 32 are typically provided on the radially inner surface of the former 1 to support the weight of the former against the bore tube 8 of the cryogen vessel, and to limit radial movement between the former and the bore tube. The remainder of the radially inner surface of the former is slightly spaced away from the radially outer surface of the bore tube 8.
The cylindrical magnet is essentially symmetrical about axis AA. References herein to “axial” and “radial” directions are determined with reference to this axis.
Also illustrated in FIGS. 1A-1B are an outer vacuum container 4 and thermal shields 3. As is well known, these serve to thermally isolate the cryogen vessel 2 from the surrounding atmosphere. Insulation 5 may be placed inside the space between the outer vacuum container and the thermal shield. The available inside diameter 4a of the cylindrical magnet arrangement is required to be of a certain minimum dimension to allow patient access.
The magnet assembly, comprising the coils 34 on the former 1, needs to be securely mechanically connected to the cryogen vessel 2 to prevent rotational and axial movement in service.
FIGS. 1C-1D schematically illustrate conventional arrangements for locating a magnet former 1 firmly in position relative to a bore tube 8 of a cryogen vessel 2. This is conventionally achieved by relatively complex attachment of mechanical mounting components to the former 1, which is generally made of aluminium. The mechanical mounting components are subsequently welded to the bore tube 8 of the cryogen vessel 2. The OVC bore tube 8 and the mechanical mounting components are typically of stainless steel. Known methods for attaching the magnet former to the cryogen vessel bore tube 8 include brackets screwed to the former 1, which are then welded to the bore tube 8.
FIG. 1C shows an example of a conventional arrangement. As shown, several stainless steel brackets 80 are attached to the aluminium former 1 through holes 81 provided at suitable locations. At least one threaded hole 84 is provided into the material of the former for each bracket, and a corresponding at least one bolt 82 is screwed through a hole in bracket 80 into each threaded hole 84 to retain the bracket in position. Holes 81 are dimensioned and positioned to allow access for positioning the brackets 80 and tightening the bolts 82. In position, the brackets meet a radially outer surface of the cryogen vessel bore tube 8. The brackets are then welded 86 to the outer surface of the cryogen vessel bore tube, through holes 81. The radially inner surface of the former 1 is spaced away from the radially outer surface of the bore tube 8 by support protrusions discussed with reference to FIG. 1B. The assembly process is intricate and time-consuming. Specialist welding methods must be used, requiring highly skilled labour.
This mounting process often requires significant machining operations on the former, additional components and extended assembly time, all of which add cost to the manufacture of the cylindrical magnet, and add risk of damage. There is a general tendency for cylindrical magnets for MRI and NMR systems to be made as short as possible, and as improvements are made in this area and systems get shorter, access to suitable mounting locations gets increasingly difficult, making the assembly operation yet more difficult, costly and time-consuming. Current efforts in reducing the length of magnet systems mean that the space required for the provision of access holes 81 may not be available.
Alternatively, as illustrated in FIG. 1D, split bore tubes have been employed. The cryogen vessel bore tube 8 is formed in several pieces 8a, 8b. A backing bar 90 is provided, and the pieces 8a, 8b of the cryogen vessel bore tube are welded to the backing bar to form a complete bore tube. During assembly, the backing bar 90 is located in a recess running around a radially inner circumference of the former 1. It is held in position by spring tension. Locating pins 94 are passed through locating holes 96 provided in the former for the purpose. These locating pins 94 are typically of stainless steel and 6-10 mm diameter. About 12-24 of these pins may be placed radially around a circumference of the cryogen vessel bore tube 8. These pins will fit in the locating holes 96 tightly enough to prevent significant relative movement of the former and the bore tube in the finished structure. The pins have been shown to have a loose fit in the drawing for the purpose of illustration. The locating pins have a narrowed end 93, which fits into a corresponding receiving hole 95 in the backing bar 90. When all the locating pins have been secured to the backing bar in this manner, the backing bar is retained firmly in its position by spring tension of the backing bar acting on the various retaining pins 94. The two parts 8a, 8b of the cryogen vessel bore tube are then aligned and introduced into the backing bar. A single weld 98 joins the retaining pins, the backing bar and the parts of the bore tube. The resulting bore tube is retained in its axial position by the locating pins 94, and is radially positioned by support protrusions 32 as discussed with reference to FIG. 1B. This latter solution has been found to be particularly complex and expensive to implement.
The invention provides methods and tools useful in securely attaching and axially locating a cylindrical superconducting magnet former 1 to a bore tube 8 of a cryogen vessel 2.
Among other objectives, the present invention seeks to reduce the labour costs involved in producing a cylindrical magnet structure comprising a cylindrical superconducting magnet former attached to a bore tube of a cryogen vessel.
The present invention accordingly provides methods, tooling and apparatus as defined in the appended claims.